Endlers are guppies, from what I've read they were given their own species classification to help differentiate the locality variant for the sake of preservation. Not 100% on that but it does sound plausible. They do interbreed regularly and easily, and I've not had or seen any of the deformity, sterility, and high death rate issues that is common with hybridization of more distant "relatives."
The two are interbred so commonly now that I wouldn't bother keeping an endler-only tank unless you know that the line you have can be traced back to collection. (n-class)